1. Field of the Invention
The present invention relates to a RAKE receiver having a MIXR (Multipath Interference exchange Reduction) function that reduces interference by using MICT (Multipath Interference Correlative Timing).
2. Description of the Related Art
In a RAKE receiver, the result of despreading a received signal at each of the path timings of a plurality of paths detected from the received signal contains as interference the result of despreading the received signal from another path at a timing displaced from path timing thereof. Japanese Unexamined Patent Publication No. 2003-133999 describes how the interference from other paths is reduced using MIXR. To describe this briefly, consider the case of FIG. 1 where there are a path i with timing tii and a path j with timing tjj between a transmitter and a receiver, and let X denote the value of the cross correlation between the received signal from the path j and the despreading code at the timing tii of the path i, and Y denote the value of the cross correlation between the received signal from the path i and the despreading code at a timing tij located at a position symmetric to the timing tjj with respect to the timing tii on the time axis; in this case, X is equal to Y because the timings of the despreading codes are displaced equally with respect to the respective received signals. This means that the result of the despreading performed at the timing tij contains an interference component from the path j when despreading was performed at the timing of the path i. Accordingly, if the result of the despreading performed at the timing tij is appropriately weighted and added to the result of the despreading performed at the timing of the path i, the interference component from the path j can be reduced. This timing tij is called the multipath interference correlative timing (MICT) for tii with respect to tjj, and the addition by appropriate weighting is called the MIXR combining.
Here, suppose that N paths have been detected as a result of path timing detection by a path search. As shown in FIG. 1, two timings tij and tji can be considered as the MICTs for the paths i and j. Here, MICTtij is obtained by the following equation.tij=2tii−tjj  (1)
However, if all the MICTs are to be calculated using the equation (1), the total number of MICTs and path timings will be the square of the number, N, of paths. If all of these timings are to be assigned to fingers, there arises the problem that as the number of paths increases, the amount of necessary hardware will increase prohibitively.